Method and device for continuous casting and subsequent forming of a steel billet, especially a billet in the form of an ingot or a preliminary section

ABSTRACT

A method and device for continuous casting and subsequent deformation of a cast strand ( 1 ) of steel, especially a cast strand ( 1 ) of an ingot shape ( 2 ) or a preliminary section shape, in which the secondary cooling ( 4 ) and the strand support ( 11 ) are matched to the cooling state of the cast strand cross section ( 1   a ) with respect to steel quality and in which the segregations and porosity are of significance with respect to further processing and the end uses of the product and require steps for improvement of the 10 internal quality and for the surface qualities, in which the secondary cooling ( 4 ) in its geometrical configuration is analogously matched to the solidification profile ( 5 ) of the cast strand ( 1 ) along the respective distance travelled ( 6 ) of the cast strand and in which the cast strand support ( 11 ) likewise is analogously reduced in dependence upon the solidification profile ( 5 ) of the cast strand ( 1 ) of the distance/travelled ( 6 ).

[0001] The invention relates to a method and a device for continuouscasting and subsequent shaping of a cast strand [billet] of steel,especially a cast strand with an ingot shape or the shape of apreliminary section [structural shape], in which the [geometries of the]secondary cooling and strand guide are matched to the cooling state ofthe cross section of the cast strand [billet].

[0002] In general in continuous casting for different types of steel andproducts of different dimensions or layouts, attention is directed tothe growth of the solidifying strand shell only during the secondarycooling and to the position of the molten pool [sump] tip in adeformation stretch. Thus it is known (EP 0 804 981 A) to squeeze thecast strand [billet] in the deformation stretch so that the desiredfinal thickness will result. For that purpose it is however onlyrequired to establish the position of the molten pool tip, based uponwhich the deformation force is applied horizontally along a wedgesurface. Such a process is coarse and does not take into considerationthe state of the lattice structure to be expected. The reasons lie inthe disadvantageous heat distribution by disadvantageous cooling and auniform strand bracing with nonuniform heat abstraction from the strandcross section. A matching of the secondary cooling to the strand supportlikewise is not to be found.

[0003] The invention presents as its object to so match the secondarycooling, strand support and deformation temperature to one another thateven types of steel which are also very difficult to cast, can be castand, indeed, so that all qualities of steel, in which segregations andporosities are of significance for further processing and end-usepurposes can be used and, aside from this, features improved internalqualities and surface qualities.

[0004] The object set forth is achieved according to the invention inthat the secondary cooling has its geometrical configuration matchedrespectively and analogously to the solidification profile of theimmediately following length segment of the cast strand [billet] and inthat the strand support likewise is reduced analogously depending uponthe solidification profile of the cast strand at the immediatelyfollowing length segment. The strand support can be matched to thestrand shell growth on all sides in that the roller box lengths are thesame as or smaller than the molten-pool [sump] width, whereby edgecooling is avoided. In this manner the cast material is significantlyimproved as to its lattice structure [internal structure qualities] andits surface quality.

[0005] According to a refinement, the corner regions of the cast strandcross section are less cooled with increasing travel distance [caststrand segment length] than the middle regions. The individual sides arethus cooled with reduced application of water thereto to optimize thetemperature distribution in the strand cross section, whereby asubsequent soft-reduction process can also be influenced.

[0006] According to a further refinement it is proposed that the sprayjets in the secondary cooling be so matched with their spray angles tothe strand shell thickness that as the molten-pool [sump] width becomessmaller, a smaller spray angle is used. In this way, the secondarycooling is matched using the spray angle to the strand shell growth andcreates an optimal temperature distribution in the strand cross sectionand also at the surface so that there is weaker temperature drop at theedges.

[0007] A similar effect can be brought about with a decreasingmolten-pool [sump] width in that the spacing of the spray nozzlesproducing the spray jets from the strand [billet] surface is varied independence upon the solidification profile.

[0008] A further heat withdrawal is also limited in that, in accordancewith another feature, the corner regions of the cast strand [billet]cross section with increasing travel distance is supported to a lesserextent than the middle region. The lack of contact by longer supportrollers then reduces the heat abstraction.

[0009] A further development of the feature of temperature distributionand equalization is that the corner regions and/or the side surfaces ofthe cast strand [billet] cross section are insulated against heatabstraction. The process-matched secondary cooling for producing anoptimal solidification structure is followed by a targeted thermalinsulation of the strand cross section for producing a softer strandcross section core for the soft-reduction process.

[0010] Furthermore it is provided that, in addition to insulating thecorner regions and/or the side surfaces of the strand cross section, theupper and lower sides of the strand are selectively intensively cooledwith a coolant. For this purpose especially the middle regions areconsidered so that there will be a further reduction in the molten-pool[billet] width. On the upper side of the strand and the lower side ofthe strand a cooling of the surfaces is effected to provide harder anddeformation-stiffer pressing surfaces for the soft-reduction processahead of the soft-reduction segment.

[0011] After there has been a significant equalization of thetemperature in the strand cross section over layers of the strand crosssection, it is advantageous to roll the cast strand cross section fromthe top down in accordance with the so-called soft-reduction process.

[0012] A device for continuous casting and subsequent shaping of a caststrand [billet] of steel, especially a cast strand [billet] with aningot shape, whereby the secondary cooling and the strand guiding arematched geometrically to the cast strand [billet] cross section, attainsthe objects set forth for the invention in that the secondary cooling iscarried out in dependence upon the solidification profile and thedistance traveled beginning substantially with the full strand width,and in that the secondary cooling and the strand support are soreducible in dependence upon the solidification profile within thedistance traveled that the cast strand [billet] before entry into thesoft-reduction segment is supported only at the underside of the strandacross the strand width. As a result, apart from theprocess-technological improvements an improvement in the costeffectiveness of the device is obtained by a loading-matchedconfiguration of the machine components, mechanical and thermal stressesare reduced.

[0013] To avoid excessive heat abstraction at the edges of the strandcross section it is proposed to arrange cover elements on the sidesurfaces and/or the corner regions of the cast strand cross sectionwithin the secondary cooling and the strand support.

[0014] According to a further development it is provided that the softreduction segment is formed at its beginning and end with driven driverrollers in the driver frame and that the soft reduction segment isformed from at least two roller frames with roller pairs without drives,whereby the upper frame is respectively adjustable relative to the lowerframe hydraulically.

[0015] In this manner in the soft reduction segment the soft reductioncan be carried out using a multiroller segment. A continuous convergenceproduces a continuous soft reduction process over a selectable length.The theoretical precalculation of the molten-pool [sump] thickness overthe last meter in the final solidification region is determined by asuitable convergence setting and its length.

[0016] Other features reside in that in the continuous casting movementdirection ahead of and downstream of the soft reduction segment one ormore driver frames are arranged. In this manner the cast strand [billet]can be sufficiently transported in the deformation region and thedeformation forces applied in a sufficient degree.

[0017] According to other features it is provided that upstream and/ordownstream of a straightening driver, an intensive cooling device isprovided for the upper side of the strand and the lower side of thestrand of the continuous casting strand cross section.

[0018] Several steel qualities shown in further processing by so-called“quenching” a better surface structure. In combination with the coolingupstream of the soft reduction process this effect can also be achieved.The effect of the soft reduction brought about by the mechanical units(segments, driver frames) can be supported by a so-called “thermal softreduction”. For this purpose the cast strand in the regions which arehere under consideration can be additionally treated with water in atargeted manner.

[0019] Another arrangement resides in that upstream of a soft reductionsegment, an intensive cooling device is arranged for the strand upperside and the strand lower side of the cast strand cross section.

[0020] A further configuration is provided in that the soft reductionsegment forms a unit which is shiftable in the strand continuous castingmovement direction or opposite the strand movement direction and whichis arranged ahead of one or more driver frames.

[0021] In addition it is proposed that the soft reduction segments inthe continuous casting strand movement direction be arranged downstreamof the straightening and extracting machine (the driver frames).

[0022] In the drawing embodiments of the invention have been shown whichare described below in greater detail.

[0023] It shows:

[0024]FIG. 1 a side elevational view of an arcuate continuous castingapparatus for an ingot shape with soft reduction as a first alternative;

[0025]FIG. 2a the cast strand [billet] cross section in the secondarycooling with a relatively larger molten-pool [sump] width and thinstrand shell;

[0026]FIG. 2b the same cast strand cross section with reduced spray jetwidth and reduced sump width;

[0027]FIG. 2c the same cast strand cross section with further reducedspray jet width at the strand upper side and the strand lower side andfurther reduced sump width;

[0028]FIG. 3a the continuously cast strand cross section with the strandshell thickness corresponding to FIG>2 a and wider strand support;

[0029]FIG. 3b the cast strand cross section with the strand shellthickness corresponding to FIG. 2b and reduced strand support;

[0030]FIG. 3c the strand cross section with the strand shell thicknesscorresponding to FIG. 2c and a strand support at the upper and lowersides of the strand;

[0031]FIG. 4a the cast strand cross section on conventional completesolidification without the invention and without covering the sidesurfaces.

[0032]FIG. 4b the cast strand cross section without the pressuredistribution according to the invention in the soft reduction and inwhich inclusions can develop;

[0033]FIG. 5a the continuous casting strand cross section with coveringfor a temperature distribution;

[0034]FIG. 5b the continuous casting strand cross section withtemperature distribution according to the invention in the softreduction;

[0035]FIG. 6 a side elevational view of an arcuate continuous castingapparatus for an ingot shape with soft reduction as a secondalternative.

[0036] The method of continuous casting of steel in rectangular or ingotshapes according to FIG. 1 is characterized by cooling, supporting andshaping. The continuously cast strand [billet] 1 with a cast strandcross section la comprises in the exemplary embodiment an ingot shape 2and emerges from a continuous casting mold 3 and is directly cooled in asecondary cooling. As a result it passes from arc segment A to arcsegment B, C and D each with a solidification profile 5 (FIGS. 2a, 2 b,2 c) in which an already solidified strand shell 5 a grows from arcsegment to arc segment with increasing strand shell thickness 5 b. Themethod operates so that the secondary cooling 4, in its geometricalconfiguration, is analogously matched to the solidification profile 5 ofthe cast strand 1 over the respective travel length 6 of the continuousstrand from arc segment A to arc segment D, and whereby a strand support11 also is reduced analogously as a function of the solidificationprofile of the cast strand 1 over the following travel length 6. As aresult the corner regions 1 b of the cast strand cross section la withincreasing travel length 6 are less cooled than in the central regions 1c.

[0037] This rule can be followed in that the spray jets 7 in thesecondary cooling 4 have their spray angles 7 a so matched to therespective continuous strand shell thickness 5 b that a smaller sprayangle 5 a is associated with a sump width [molten pool width] 8 whichbecomes smaller.

[0038] Alternatively, the spacing 9 of the spray nozzle 10 producing thespray jets 7 from the strand upper surface 1 d is reduced as a functionof change in the hardening profile 5 (FIG. 2b).

[0039] In this sense, the corner regions 1 b of the cast strand crosssection 1 a are supported to a lesser extent than the middle regions 1 cwith increasing travel length 6 (FIGS. 3a, 3 b, 3 c).

[0040]FIGS. 4a and 4 b show completely solidified cast strand 1 alargely uniform temperature distribution in its outer regions, wherebyundesirable indentations 18 can form (FIG. 4b).

[0041] For a uniform heat distribution in a form for subsequentdeformation processing, the corner regions 1 and/or the side surfaces 1e of the cast strand cross section 1 e are insulated against heatabstraction (FIGS. 5a and 5 b). As a result temperature boundary regions19, 20 and 21 are formed. In the middle of the cast strand cross section1 a the temperature boundary regions 21 prevails (FIG. 5b) in whichdeformation work by pressing from above downwardly is promoted. In thiscentral region the temperature is higher than above and below it and asa result segregations are easily dispersed and porosity eliminated inthis way.

[0042] In addition to insulating the corner regions 1 b and or the sidesurfaces 1 e, the cast strand cross section 1 a is selectivelyintensively cooled with a coolant at the strand upper side 1 f and thestrand lower side 1 g.

[0043] In a further process step, the strand cross section 1 a is rolledfrom top to bottom by the so-called soft reduction method whereby anotherwise customary squeezing does not occur.

[0044] The illustrated device for continuous casting and subsequentshaping of a cast strand 1 of steel especially are cast strand 1 with aningot 2 with a secondary cooling 4 and the strand support 11 is matchedto the cooling state of the cast strand cross section 1 a is so shapedthat the secondary cooling 4 as a function of the solidification profile5 and the set back travel length 6, commences with substantially thefull strand width 1 a, the secondary cooling 4 and strand support 11being reduced, depending upon the solidification profile 5 of the caststrand 1 within the travel length 6 for such that the cast strand 1before entry into a soft reduction segment 12 is supported only at theunderside 1 g of the strand width 1 h. In order to bring about thedesired temperature distribution with a deformable layer in the middle,within the secondary cooling 4 and the strand support 11, on the sidesurfaces 1 e of the cast strand cross section 1 e and/or on the cornerregions 1 b, cover elements 13 are arranged which can form the anglepieces 13 a.

[0045] The soft reduction segment has at its start 12 a and its end 12b, driver frames 14 with driven drive rollers 14 a. The soft reductionsegment 12 is comprised itself of two or more roller frames 12 c whoseroller pairs are without drives. An upper frame 12 d is hydraulicallyadjustable relative to the lower frame 12 e.

[0046] One or more driver frames 14 are arranged in the strand movementdirection 15, in addition, upstream and downstream of the soft reductionsegment 12.

[0047] In order to produce the desired temperature distribution inhorizontal through-hardened layers upstream of a soft reduction segment12 an intensive cooling device 17 is arranged for the strand upper side1 f and the strand lower side 1 g of the cast strand cross section 1 a.This raises the strength and forms a soft reduction preparation. Theintensive cooling on the strand upper side 1 f and the strand lower side1 g can be provided not only ahead of the levelling driver 16, but alsoupstream of the controllable soft reduction segment 12 or downstream ofthe straightening driver 16.

[0048] In FIG. 6 a second alternative configuration is shown.

[0049] There the soft reduction segment 12 is configured as a shiftableunit 12 f which can be displaced in the strand movement direction 15 oropposite the strand movement direction and in the strand movementdirection is arranged upstream of one or more driver frames 14.

[0050] The soft reduction segment in the straightening driver region isconceived as a compulsory concept in conjunction with the extractionconcept in the case of ingot plants generally having two straighteningpoints. Because of the elastoplastic properties of the material in abending-straightening process, the cast strand 1 develops a straightconfiguration. By contrast with slab plants in which the strand istransitioned to a straight shape via a curved path, the ingot strand inthe straightening region has a bend line which depends upon suchinfluencing parameters as the moment of inertia, the temperature of thecast strand and the temperature distribution within the cast strandcross section and which may differ depending upon the straighteningpoint over short stretches from the basis bend line and the bendingturning point so that the cast strand in this region may have especiallystrong creep properties. Based upon a predetermined curved path in thesoft reduction segment 12, a permissible elongation element can beprovided in practice. Through the elastoplastic properties produced bythe bending properties, the cast strand 1 can be brought into a state(determined in terms of the theoretical elastic limit, the flowproperties or the like which in a normal case can yield a reduced forcecost for additional soft reduction.

[0051] Reference Character List

[0052]1. Cast strand [billet]

[0053]1 a. Cast strand cross section

[0054]1 b. Corner region

[0055]1 c. Middle region

[0056]1 d. Strand surface

[0057]1 e. Side surfaces of the cast strand cross section

[0058]1 f. Strand upper side

[0059]1 g. Strand lower side

[0060]1 h. Strand width

[0061]2. Ingot shape

[0062]3. Continuous casting mold

[0063]4. Secondary cooling

[0064]5. Solidification profile

[0065]5 a. Strand shell

[0066]6. Travel length

[0067]7. Spray jet

[0068]7 a. Spray angle

[0069]7 b. Spray jet width

[0070]8. Molten-pool [sump] width

[0071]9. Spacing

[0072]10. Spray nozzles

[0073]11. Strand support

[0074]12. Soft reduction segment

[0075]12 a. Start

[0076]12 b. End

[0077]12 c. Roller frame

[0078]12 d. Upper frame

[0079]12 e. Lower frame

[0080]12 f. Shiftable unit

[0081]13. Cover elements

[0082]13 a. Angle pieces

[0083]14. Driver frame

[0084]14 a. Driver rollers

[0085]15. Strand

[0086]16. Movement direction

[0087]17. Intensive cooling device

[0088]18. Indentation

[0089]19. Temperature boundary region

[0090]20. Temperature boundary region

[0091]21. Temperature boundary region

1. A method of continuously casting and then deforming a cast strand ofsteel, especially a cast strand with an ingot shape or preliminarysection shape, in which the secondary cooling and strand guidance arematched to the cooling state of the cast strand cross section,characterized in that the secondary cooling in its geometricalconfiguration is matched analogously to the solidification profile ofthe cast strand of the following traveling length of the cast strand andthe strand support also is analogously reduced as a function of thesolidification profile of the cast strand at the respective travellength.
 2. The method according to claim 1 characterized in that thecorner regions of the cast strand cross section with increasing travellength are less cooled in the middle regions.
 3. The method according toone of claims 1 or 2 characterized in that the spray jets in thesecondary cooling have their spray angles so matched to the strand shellthickness that a smaller molten pool width is juxtaposed with a smallerspray angle.
 4. The method according to one of claims 1 through 3characterized in that the spacing of the spray nozzles producing thespray jets from the strand surface is varied as a function of thesolidification profile.
 5. The method according to one of claims 1through 4 characterized in that the corner regions of the cast strandcross section are supported to a lesser degree than the central regionwith increasing travel length.
 6. The method according to one of claims1 through 5 characterized in that the corner regions and/or the sidesurfaces of the cast strand cross section are insulated against heatabstraction.
 7. The method according to one of claims 1 through 6characterized in that in addition to the insulation of the cornerregions and/or the side surfaces of the strand cross section, the strandupper side and the strand lower side are selectively intensively cooledwith a coolant.
 8. The method according to one of claims 1 through 7characterized in that the cross strand cross section is rolled from thetop downwardly according to the so-called soft reduction method.
 9. Adevice for continuous casting and then deforming a cast strand of steel,especially a cast strand with an ingot shape whereby the secondarycooling and the strand guidance is matched to the cooling state of thecast strand cross section characterized in the secondary cooling (4) independence upon the solidification profile and in that the travel length(6) beginning substantially with the full strand width 1 h is extractedand that the secondary cooling (4) and the strand support (11) are soreduced as a function of the solidification profile (5) of the caststrand (1) within the travel length (6) that the cast strand (1) beforeentering the soft reduction segment (12) is only supported on the strandunderside (1 g) of the strand width (1 h).
 10. The device according toclaim 9 characterized in that within the secondary cooling (4) and thestrand support (11) cover elements (13) are arranged on the sidesurfaces 1 e of the cast strand cross section (1 e) and on the cornerregions (1 b).
 11. The device according to one of claims 9 or 10characterized in that at the start (12 a) and end (12 b) of the softreduction segment (12) drive frame (14) with driven driver rollers (14a) are provided and that the soft rejection segment (12) is comprised ofat least two roller frames (12 c) with roller pairs without drives andthe upper frame (12 d) can be adjustable hydraulically with respect tothe lower frame (12 e).
 12. The device according to one of claims 9-11characterized in that one or more driver frames (14) are arranged in thestrand movement direction (15) upstream and downstream of the softreduction section.
 13. The device according to one of claims 9-12characterized in intensive cooling device (17) is arranged upstreamand/or downstream of a straightening driver for the strand upper side (1f) and the strand lower side (1 g) of the cast strand cross section (1a).
 14. The device according to one of claims 9-13 characterized in thatupstream of a soft reduction segment (12) an intensive cooling device(17) is arranged for the strand upper side (1 f) and the lower side (1g) of the cast strand cross section (1 a).
 15. The device according toone of claims 9-14 characterized in that the soft reduction segment (12)forms a unit (12 f) shiftable in the strand movement direction (15) oropposite the strand movement direction (15) and is arranged upstream ofone or more drive frames (14).
 16. The device according to one of claims9-15 characterized in the soft reduction segment (12) is arranged as astraightening and soft reduction segment between the driver frames (14).17. The device according to one of claims 9-16 characterized in that thesoft reduction segment (12) is arranged in the strand movement direction(15) downstream of the straightening and extraction machine.